Analogue and digital instruments for non-invasive estimation of muscle fibre conduction velocity

Analogue and digital instruments for non-invasive on-line measurement of muscle fibre conduction velocity (CV) have been designed, built and compared using test signals and real myo-electric signals. Their inputs consist of two single-differential or double-differential myo-electric signals, obtained using a three- or four-contact surface electrode system. The analogue device computes CV by tracking the lag of the zero-crossing of the cross-correlation between the first signal and the derivative of the second. The digital device computes the peak of the cross-correlation function between the two signals by sampling them at 50 KHz for 20 ms (or longer, up to 320 ms), computing CV in about 30 ms (or longer, up to about 670 ms) and resuming sampling. Both devices allow estimation of CV during either voluntary or electrically elicited contractions and include a stimulation stage and a signal conditioner with artefact suppression features. Both devices provide analogue and numerical outputs and allow interfacing with analogue and digital instrumentation. They can be used in clinical or in research environments for easy and quick identification of appropriate electrode locations and/or for monitoring CV during sustained voluntary or electrically elicited contractions. The digital version is more versatile and requires no adjustments; it provides an estimate based on intermittent reading of the signals and is more sensitive to noise and momentary CV fluctuations.